1. Field of the Invention
The present invention relates to an amplifier circuit and, more particularly, to a low-frequency negative-feedback amplifier circuit using an operational amplifier.
2. Description of the Related Art
In general, a general-purpose amplifier, such as an operational amplifier (hereinafter referred to as an op amp), has a constant open-loop voltage gain (hereinafter referred to as Gvo) even if its closed-loop voltage gain (hereinafter referred to as Gv) is selected to be an arbitrary value when it is actually used.
FIG. 7 illustrates such an op amp, in integrated-circuit form, which is used as a noninverting amplifier.
An op amp A7 is housed in a package 51, which is equipped with terminals 1 and 5 adapted to adjust an offset voltage, a terminal 2 serving as the inverting input terminal of the op amp, a terminal 3 serving as the noninverting input terminal of the op amp, a terminal 4 connected to a negative power supply (-Vcc) or ground potential (GND), a terminal 6 serving as the output terminal of the op amp, a terminal 7 connected to supply voltage (Vcc) and a preliminary terminal 8.
The terminal 3 of the package 51, which is connected to the noninverting input terminal of the op amp A7, is connected to the sliding contact of a variable resistor VR71 adapted to vary the level of an input signal. The variable resistor VR71 has its end connected to an input terminal IN and its other end connected to ground. The terminal 2 of the package, which is connected to the inverting input terminal of the op amp A7, is connected to an output terminal OUT via a feedback resistor R71 and to ground via a series combination of a feedback capacitor C71 and a feedback resistor R72. The above-mentioned Gv is determined by a ratio of R71 and R72.
In the above amplifier, Gvo is constant, while Gv may be set arbitrarily by users. For this reason the following problems will arise.
First, in the case where Gv is set relatively high, say, 70-80 dB when Gvo=100 dB (constant), a state akin to the state without feedback will result. In this case, therefore, distortion will increase and frequency characteristics will deteriorate.
Second, if Gv is set relatively low, say, 0-10 dB when Gvo=100 dB, stability decreases and thus oscillation is apt to occur.
In order to solve the first problem, therefore, it is required that Gvo should be further increased relative to Gv. Also, in order to solve the second problem, it is necessary that Gvo be further decreased relative to Gv.
In the prior art, as shown in FIG. 7, the high-impedance variable resistor VR71 is connected to the terminal 3, serving as the noninverting input terminal of the op amp, so as to adjust the input signal level.
With such an arrangement, however, the impedance seen by the input terminal becomes necessarily high and moreover the variable resistor itself serves a source of noise because it involves thermal noise. The noise level is at a maximum when the sliding contact of the variable resistor VR71 is set at its middle point.